The new findings follow a number of recent studies that have
suggested such stem cells exist in adult mice, and can give rise
to healthy offspring in animals that have had their fertility
destroyed by chemotherapy. However, these studies have been
controversial, because they go against years of research
suggesting otherwise, experts say.

In the new study, the researchers devised a more rigorous way to
isolate these cells, and for the first time, suggested their
existence in people.

If true, the findings could have implications for women's
fertility treatments. Currently, women who choose to undergo in
vitro fertilization (IVF) for infertility must endure hormone
injections so doctors can retrieve eggs for fertilization, said
study researcher Jonathan Tilly, director of the Vincent Center
for Reproductive Biology at Massachusetts General Hospital. But
if researchers could isolate egg-producing stem cells from
ovaries, it might be possible to conduct that whole process
outside the body, Tilly said.

In the new study, Tilly and colleagues isolated egg-producing
stem cells from human ovary tissue by targeting a protein found
on the surface of only these cells. In dishes, the cells grew
into cells that had properties of human eggs. For instance, they
had half the genetic material of other cells in the body.

Next, to show the stem cells could produce eggs, the researchers
placed a gene into the stem cells that made them glow green,
placed the stem cells into human ovarian tissue (taken during a
biopsy), and grafted this tissue into mice. One to two weeks
later, this tissue contained egg cells glowing green, showing
they had formed from the stem cells, the researchers said.

The researchers don't yet know if these egg cells could be
fertilized to produce children. The United States does not allow
human eggs to be fertilized for research purposes. The
researchers also don't know whether these egg-producing stem
cells are active throughout a woman's life, or only when they
receive a particular signal, Tilly said, although the researchers
have a follow-up study planned to address this question.

The number of egg-producing stem cells appear to be quite minute.
In mice, they make up about 0.014 percent of all cells in the
ovary, Tilly said.

Still a Controversy

"It's very novel and it's very exciting," said Dr. Sandra Carson,
professor of obstetrics and gynecology, at Brown University's
Women & Infants Hospital, who was not involved in the study.

"It certainly makes sense that there would be those stem cells
still there," said Carson, noting men have stem cells that
produce sperm throughout life.

However, other researchers say the new paper does not resolve the
controversy of whether egg-producing cells exist in adult
ovaries.

"I would like to see better characterization of this very small
pool of cells that may be present in the ovary," said Dr. Marco
Conti, professor and director of the Center for Reproductive
Sciences at the University of California, San Francisco. Conti
noted that some properties of the egg-producing cells described
in this study do not match descriptions from previous studies.

And the paper still does not address whether these cells have any
role in adult humans.

"There is no real functional evidence that this pool of cells
indeed contributes to [egg formation] in the adult," Conti said.

But if these cells do in fact work in the way the researchers
suspect, it might be possible to grow and mature them in an
environment that resembles an ovary, Carson said.

In addition, unlike human eggs, these stem cells can be frozen
without damage, Tilly said, so it may be possible to store them
for future use.

Tilly is a co-founder of OvaScience, Inc, which has licensed the
commercial potential of these findings for development of new
fertility-enhancing procedures.